The present invention relates to a polarizer used in liquid crystal display devices, optical recording devices, optical sensors, and the like. More specifically, the present invention relates to a thin-film structure having polarization characteristics needed for polarizers and a method for making the same.
A polarizer is an optical element that extracts polarized light having a specific orientation from light containing multiple polarizations. Various types of polarizers have been proposed and implemented in practice. Examples include: polarized glass, in which column-shaped silver particles having high aspect ratios is dispersed in glass; polarizers in which island-shaped metal layers are alternated with dielectric layers and then drawn; polarized film, in which polymer is drawn and aligned; and stacked polarizers, in which dielectric films and metal films are stacked in an alternating manner and light is entered from the cross-section direction of the film.
In the field of liquid crystal technology, technical advances are being made in making optical systems more compact, light-weight, high-luminance, and the like. The popularity of liquid crystal display devices in applications such as business data display, home-theater video display and the like has been increasing rapidly. In particular, there have been significant advances in the increasing of luminance through the increasing of lamp luminance, the improving of light usage efficiency through polarization conversion and the like.
However, this has led to the problem of higher temperatures in the optical system resulting from greater compactness, higher luminance, and the like. As a result, there is a growing demand for heat-resistant properties in the optical components.
Liquid crystal display devices generally use a polarizing plate formed from an organic film with dye, as in Japanese laid-open patent publication number 2002-296417. However, with these polarizing plates with organic film, the heat resistant properties are low because of the use of organic material. Also, there are polarizing films that are dye-based and that have high heat resistance properties. However, these polarizing films have narrow working wavelength bands, thus imposing restrictions on use.
In order to overcome these problems, the use of wire-grid type polarizers has been proposed. In wire-grid type polarizers, the wire is metal and the substrate is glass so that only non-organic material is used. Thus, unlike organic materials such as the dye-based polarizers, high heat resistance properties can be provided. The wire-grid polarizers with the structures shown in U.S. Pat. No. 6,108,131 and U.S. Pat. No. 6,122,103 achieve these objects.
However, production of wire-grid polarizers requires high-precision control of wire thickness, distances between wires, and the like. In particular, production of a visible-light wire-grid polarizers require ultrafine structures where the width of a wire and a gap must be 160 nm or less. As a result, manufacturing technologies such as dry etching, photolithography, and the like must be used. The equipment needed for these technologies is costly and a large number of steps is needed, thus increasing production costs.
While the production of the polarizers described above all require complex steps, there have been attempts at forming a film having polarization splitting properties on a base using just a deposition process. In Japanese laid-open patent number Hei 4-218662, a metal (Al) and a transparent dielectric (SiO2) are vaporized simultaneously on a flat substrate from facing diagonal directions to form a structure in which plate-shaped Al and SiO2 are adhesed together and projected from the substrate. The adhesed plates have an average thickness of approximately 5 nm, a width of approximately 30 nm, and a height of approximately 1500 nm. This plate-shaped body is oriented roughly perpendicular to the direction from which vaporized particles come flying so that superior polarization characteristics are provided for visible light.
However, the results of follow-up tests conducted by the inventors showed that the structure of the plate-shaped body was near the surface of the substrate was completely different from that of the area around the upper end. Near the substrate, the metal formed fine particles and no plate-shaped structure is formed near the substrate. Furthermore, while plate-shaped structures are formed from approximately 50 nm away from the substrate surface, the cross-section shape of the plate-shaped metal changes as the distance from the substrate increases, and a widening along the thickness axis was observed. More specifically, it was not possible to obtain the film structure that was disclosed. Also, the extinction ratio of the polarizing film is approximately 5 db. This provides inadequate performance for a polarizer and prevents the technology from being used for optical communications, liquid crystal projectors, and the like.